(1) Field of the Invention
The present invention relates to a controller and control system for an underwater vehicle; specifically, a controller and control system which utilize non-linear dynamics supported by underlying mathematics to control the propulsors of an underwater vehicle.
(2) Description of the Prior Art
Future underwater platforms are expected to have numerous sensors and performance capabilities that will mimic the capabilities of aquatic animals. A key component of such platforms would be their controller. Because such platforms, and an existing U.S. Navy Biorobotic Autonomous Undersea Vehicle (BAUV) is an example, keep station in highly disturbed fields near submarines or in the littoral areas, it is essential for the platforms (vehicles) to have quick-responding controllers for their propulsor systems.
Hydrodynamic models based on conventional engineering controllers have not been able to produce the desired levels of control. Thus, a biology-inspired controller is a realistic alternative. Because the brains of animals perform complex tasks which rely on nonlinear dynamics, the underlying mathematics provide a foundation for the controller and control system of the present disclosure.
Traditional control systems are designed using linear models obtained by Jacobian linearization. This linearization allows design using frequency domain techniques (such as lag-lead compensation, PID feedback, etc.) and a state-space approach (linear optimal control, pole assignment, servo-regulation, adaptive control, etc.). However, any controller designed using linearized models of the system will fail to stabilize unless the perturbations are small.
One must use nonlinear design techniques if the control system is to operate in a larger region. For underwater vehicles, linear and nonlinear control systems based on pole placement, feedback linearization, sliding mode control, and adaptive control, etc. have been designed. However, in these designs, it is assumed that the vehicle is equipped with traditional control surfaces. As such, these vehicles have limited maneuvering capability.
For large and agile maneuvers, traditional control surfaces are inadequate and new control surfaces must be developed. Observations of marine animals provide the potential of fish-like oscillating fins for the propulsion and maneuvering of autonomous underwater vehicles (AUVs). AUVs exist with multiple oscillating fins which impart high lift and thrust. The oscillatory motion of the fins or propulsors is obtained by inferior olives which provide robust command signals to controllers and servomotors of the fins.
Inferior olives have complex nonlinear dynamics and have robust and unique self-oscillation [(Limit Cycle Oscillation (LCO)] characteristics. Efforts have been made to model the inferior olives (IO). Limited results on phase control of IOs in an open-loop sense are available using a pulse type stimulus. However, the required pulse height of the input signal which depends on the state of the IOs at the switching instant as well as the target relative phase between the IOs has not been derived. For the application of the IOs to the AUV, closed-loop control systems must be developed for the synchronization and phase control of the IOs.